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Lightwire claims silicon photonics advance

27 Feb 2008

US startup unveils details of its silicon photonics technology and announces its first product.

Lightwire aims to build optical components out of silicon using standard CMOS fabrication, integrating the optical and electronic functions on the same chip and taking advantage of the high-volume, low cost manufacturing techniques already developed for the electronics side of things. Like other developers of silicon photonics, Lightwire is targeting applications in high-speed interconnections, in the regime where copper and traditional optoelectronics technologies run into power consumption, signal integrity and cost limitations.

The first product, an SFP+ optical module, is a high-density plug-and-play form factor designed to connect servers and network equipment. The 10GBase-LRM version has particularly demanding signal integrity requirements, as it must send data over distances up to 220 m over multimode fibre.

Lightwire says the secret lies in its optical modulator structure. The vendor uses a proprietary design called SISCAP, which is based on a metal-oxide semiconductor (MOS) capacitor made from polysilicon and silicon-on-insulator (SOI).

Light is guided along the junction between the two layers, which happens to be where the peak electrical charge in the capacitor builds up. "We deliver the maximum charge at the area of maximum optical intensity, to give the maximum refractive index change in the modulator," explained Lightwire CEO Vijay Albuquerque. The refractive index change is used to switch the light.

This highly efficient interaction yields a very short device length. Lightwire says its modulator measures just 0.5 mm long, roughly the same as the device announced by IBM in November 2007 which has yet to be commercialized, and an order of magnitude smaller than those from other vendors.

Lightwire claims that its product has the lowest power consumption in the industry. The power dissipation of its SFP+ optical module is 0.4 W versus 7 W for copper cables and 1-2 W for optical 10 Gbit/s interconnects. "You need to have very high signal integrity, combined with low power consumption and low cost," noted Albuquerque. "When you put that all together with the CMOS solution that we have, then you have a technology that's very low cost in volume,"

A lot of work was put into the waveguide to reduce the optical loss, Albuquerque added, and this low loss in combination with the high efficiency in the optical modulator is responsible for the low power consumption. The modulator only requires 30 mW, which leaves plenty of power left over to add signal processing and conditioning circuitry that can be used send the optical signal further. The nanotapers used to get light on and off the chip were another area requiring significant development.

"Cost comes in two areas," said Albuquerque. "One is the cost of the devices, the other is the assembly cost. We have a very low cost and manufacturable scheme for optical coupling."

Lightwire has taken a while to get here. Founder Kal Shastri, who also invented the first integrated CMOS SERDES, started working on silicon photonics in 2001. His company OptronX foundered in the telecoms downturn and was sold to JDS Uniphase in 2003, but the silicon photonics intellectual property was retained for Shastri to transfer to his next startup, SiOptical. Last November SiOptical changed its name to Lightwire, to reflect what it was doing, and to avoid confusion with another company, CyOptics, located nearby.

Although there are relatively few companies working on silicon photonics, Lightwire finds itself up against some big names - both IBM and Intel have announced significant developments. Startups in this space include Kotura and Luxtera.

But competitor Luxtera says it's not as far behind as LightWire makes out. "I think their claims are similar to our claims," commented vice president of marketing Marek Tlalka. Luxtera's first product, a quad SFP form factor that delivers four channels of 10 Gbit/s, has a power consumption of 2.4 W or 0.6 W per channel. "A lot of power in our quad module is not really taken by the chip, it's down to voltage regulation," said Tlalka. "If we could connect our chip to the power supply the correct way, then we could get under 0.5 W per 10 Gbit/s channel. I think we're close to achieving that."

Luxtera's own secret sauce is in the detector. "As far as I know, we're the only vendor on the planet that can build photoreceivers out of CMOS," Tlalka contended.

But Tlalka does concede that at several millimetres in size, Luxtera's modulator could place a limitation on the degree of integration in the future. "Size will become very important for silicon photonics when you start to integrate optics with lots of digital logic."

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